LEDs lead to change for some silver

OTTAWA – Adding a touch of silver can improve several photonic
endeavors, such as surface-enhanced Raman scattering, surface-enhanced fluorescence
and surface plasmon resonance spectroscopy. The metal amplifies the otherwise minute
emissions that must be recorded in Raman and plasmonic studies.

Initial studies focused on solid substrates made of silver, which
were used to increase the signal of particles laid atop them. Later research broke
up these metal films into smaller and smaller segments. Eventually, nanoscale silver
particles became the target of interest, brought into proximity with materials that
needed a localized push into higher emissions.

The optical properties of silver nanoparticles determine their
functionality and applicability to whatever task may be required of them but, in
turn, these properties depend largely on their size and shape.

There are a number of thermochemical ways to synthesize silver
nanoparticles of various shapes, but the reducing agents and high heat they often
require can be too environmentally harsh for some. Photochemical methods also exist
– and are better for the environment – but they don’t offer much
control over the final size of the nanoparticles. Now, Juan C. Scaiano and his colleague
Kevin G. Stamplecoskie, of the University of Ottawa, have devised a nearly all-photonic
method of creating a variety of shapes and sizes of silver nanoparticles using nothing
but LEDs.

Scaiano and Stamplecoskie started by photochemically synthesizing
“seed” particles and growing them to about 3 nm. They irradiated batches
of the seed in solution with LEDs of specific emission wavelengths and found that
a dose of 405-nm radiation caused the seed particles to grow and aggregate. Using
455-nm light, they found, formed dodecahedral masses; 505-nm light, hexagonal platelets
interspersed with a few dodecahedral particles; 590-nm or 627-nm light, roughly
triangular platelets; and 720-nm light, rods with an aspect ratio of about 2:1 or
3:1. Each change in shape corresponded with a change in the absorption spectra of
the amassed particles.

The scientists similarly tested the seed particles using a laser
and a xenon lamp. The lamp took far longer to provide results (longer than one week
versus 1 to 48 h for the LEDs). Also, Scaiano said, the resulting products “were
more of a distribution of shapes that were aggregated more if they formed at all.”

Researchers at the University of Ottawa have devised an LED-based approach to growing and controlling
the shape of silver nanoparticles. The various shapes result in different emission
wavelengths. Courtesy of the American Chemical Society.
A pulsed laser gave no result in changing particle size, and although
a high-power continuous-wave laser might work – if its beam were made divergent
– it would be too costly in comparison with the LEDs.

According to the investigators, the LED wavelength and the localized
electromagnetic field that surrounds the seed particles as they are irradiated are
the driving forces for changes in the nanoparticles’ final shape.

Next, the team will be working to implement the various particle
shapes into applications in catalysis and spectroscopy, while examining how each
shape and exposed crystal facet affects results.

“The biggest advantage [of the LED-based technique] is that
you can obtain many different sizes and shapes that all have similar surface functionalization,”
Scaiano said. “Therefore, when doing something like catalysis, these particles
do not have different ligands on the surface, which is commonly the case and which
can adversely affect the catalysis.”

Scaiano and Stamplecoskie report their work in the Feb. 17, 2010,
issue of the Journal of the American Chemical Society.